tsnet: add test for packet filter generation from netmap

This is an integration test that covers all the code in Direct, Auto, and
LocalBackend that processes NetMaps and creates a Filter. The test uses
tsnet as a convenient proxy for setting up all the client pieces correctly,
but is not actually a test specific to tsnet.

Updates tailscale/corp#20514

Signed-off-by: James Sanderson <jsanderson@tailscale.com>

ipn/ipnlocal/local.go

@@ -1510,7 +1510,11 @@ func (nb *nodeBackend) peerCapsLocked(src netip.Addr) tailcfg.PeerCapMap {
 	return nil
 }
 
-func (nb *nodeBackend) GetFilterForTest() *filter.Filter {
+func (b *LocalBackend) GetFilterForTest() *filter.Filter {
+	if !testenv.InTest() {
+		panic("GetFilterForTest called outside of test")
+	}
+	nb := b.currentNode()
 	return nb.filterAtomic.Load()
 }